In certain types of laser apparatus, for example multi-pass and regenerative amplifiers, there is provided at least one stage of amplification through which a laser beam is passed two or more times. In such systems, energy is injected from an oscillator or a previous amplifier stage. As the laser beam propagates through the amplifying medium, its energy is increased by an amount equal to the instantaneous single pass gain. Eventually, if enough passes are encountered, the energy of the resultant laser beam may approach the damage threshold within the laser apparatus itself. Back reflections from any source may also cause severe damage. For example, damage may be caused to the optics in front of the multi-pass or regenerative amplifier by the leakage of light through the injection optics and back towards the previous amplifier stages and/or towards the oscillator. Furthermore, light back reflected from targets irradiated by the laser beam may also cause damage if allowed to propagate too far back through the laser system.
In order to provide the required protection, it is known to provide isolators within the laser apparatus which allow one way passage of light.
Commonly used active devices for achieving this objective include Pockels cells which work on the electro-optic effect and Faraday rotators which employ the magneto-optic effect. In the Pockels cell, phase changes are produced in polarised light passing through a crystal made birefringent by means of stresses induced by an external electric field. By adjusting the field strength, it is possible, when desired, to rotate the plane of polarisation of the light emerging from the Pockels cell such that the polariser reflects most of the laser light out of the apparatus rather than transmitting it to subsequent optical components.
In this respect, Pockels cells or Faraday rotators can act as fast shutters or as one way light valves for controlling the propagation of laser light. However, complex electronics must be employed for controlling the Pockels cell or Faraday rotator in order that it will be able to rotate the plane of polarisation of laser light passing therethrough to the required degree and, for a Pockels cell, at the required time.
Quarter-wavelength plates (or Rhomb prisms) may also be employed in conjunction with polarisers for rotating the polarisation plane of linearly polarised light so as to extract it from double passed laser apparatus by means of the polariser.
There exists an inherent drawback in devices which rely on the relative retardation between the field vectors of the polarised light passing therethrough, in that they are not 100% efficient. Consequently, a fraction of the laser light may remain within the laser apparatus and the energy of this fraction may be sufficiently high to cause damage. This problem may be exacerbated when spurious polarisation changes occur within the laser apparatus.
Such polarisation changes may be caused, for instance, on account of the fact that the flashlamps which are used for pumping the laser are inefficient and much of the flashlamp energy is lost as heat in the laser medium itself. To prevent the heat from damaging the laser medium, the laser medium is externally cooled and so there obtains a lateral temperature gradient through the medium whereby it is hotter along its centre than around its periphery. The temperature differential thus obtained causes thermal stresses to be generated within the medium which vary spatially along its thickness. This, in turn, produces birefringence whereby the refractive index across the thickness of the medium varies according to the polarisation plane of the laser light.
As a result, when such a laser medium is interposed between a retro-reflecting surface and a 1/4-wavelength plate plus polariser, the retro-reflected light extracted from the 1/4-wavelength plate will not be rotated by exactly 90.degree. and this produces both losses and, more importantly, creates leakage through the polariser. This leakage may be sufficient to damage the oscillator or other components between the oscillator and the multi-pass or regenerative amplifier.
Various alternative isolators and optical power limiters have been suggested in the literature. For example, Natarov et al. in Sov. J. Quantum Electron. 14(6), June 1984, page 871, describe a "specific configuration of a four-pass laser amplifier with a stimulated Brillouin scattering mirror". The stimulated Brillouin scattering (SBS) mirror is a threshold device which transmits light whose intensity is below a predetermined threshold but reflects light when its intensity exceeds that threshold. In the system disclosed by Natarov et al., an SBS mirror is employed which transmits unfocused subthreshold intensity laser light produced by a suitable oscillator and transmitted through a conventional isolation device. The beam is amplified in a laser rod, retro-reflected by a concave mirror and re-amplified by the laser rod. The focus of the concave mirror is adjusted to be within the SBS cell. On encountering the SBS medium a second time, the focused and twice amplified beam is partially retro-reflected towards the amplifier rod and the beam retraces its path through the system. The light propagates through the SBS cell a final time and is fully transmitted because, although now amplified four times, it is unfocused. The conventional isolator then extracts the beam and prevents it from reaching the oscillator. Natarov et al. show that the maximum reflection coefficient of the focused beam in the SBS mirror is approximately 60% and its transmission is consequently 40% even when the beam's energy exceeds the threshold. Consequently, the SBS medium employed by Natarov et al. is used only as a mirror and never as an isolator.
M. J. Soileau et al., in IEEE Journal of Quantum Electronics, Vol. QE-19, No. 4, April 1983, describe an optical power limiter for use with a mode-locked Nd:YAG laser employing the self-focusing non-linear refraction characteristics associated with liquids which may or may not exhibit stimulated Brillouin scattering. Such an arrangement permits a laser beam to pass through the isolating medium such that its output is so de-focused as to be incapable of causing optical breakdown. In this respect, it is not truly an "isolator" and is not suitable for use in laser amplification devices from which it is desired to extract a high energy, good optical quality laser beam whilst, at the same time, preventing consequent damage to neighbouring components.
Likewise, Thomas F. Boggess, Jr. et al. ("Optical Limiting in GaAs", IEEE Journal of Quantum Electronics, Vol. QE-21, No. 5, May 1985) discuss a non-linear optical limiter for protecting sensitive optical components from high-power laser radiation and optical transients. The optical limiter described by Boggess et al. is similar, in principle, to that described by Soileau above and employs non-linear self-focusing through the non-linear medium in order to increase the effective aperture of the output beam and to reduce its intensity accordingly.
The optical limiting devices disclosed by both Soileau and Boggess are essentially the optical equivalents of a Zener diode and can be used as such to smooth intense optical transients. Thus, whilst acting as effective optical limiters, they do nevertheless permit a predetermined fraction of the input beam to pass therethrough. Preferably, an ideal optical isolator would substantially reduce the intensity of a laser beam whose power exceeded a predetermined threshold.